
/*
Copyright (c) 2019 The Khronos Group Inc.
Use of this source code is governed by an MIT-style license that can be
found in the LICENSE.txt file.
*/


#ifdef GL_ES
#extension GL_OES_standard_derivatives : enable
precision mediump float;
#endif

// setting a boundary for cases where screen sizes may exceed the precision
// of the arithmetic used.
#define SAFETY_BOUND 500.0

// Macro to scale/bias the range of output.  If input is [-1.0, 1.0], maps to [0.5, 1.0]..  
// Accounts for precision errors magnified by derivative operation.
#define REDUCE_RANGE(A) ((A) + 3.0) / 4.0


uniform float viewportwidth;
uniform float viewportheight;

varying vec2 vertXY;

void main (void)
{
	const float M_PI = 3.14159265358979323846;
	float func;

	if( (gl_FragCoord.x < SAFETY_BOUND) && (gl_FragCoord.y < SAFETY_BOUND) )
	{
		// average of horizontal and vertical abs cosine waves with periods of 128 pixels

#ifdef GL_OES_standard_derivatives
		func = REDUCE_RANGE(0.5 * (abs(cos(fract(gl_FragCoord.x / 128.0) * (2.0 * M_PI))) + abs(cos(fract(gl_FragCoord.y / 128.0) * (2.0 * M_PI)))));
#else
    func = 0.5;
#endif

		gl_FragColor = vec4(func, func, func, 1.0);
	}
	else discard;
}

